1. Field of the Invention
The present invention relates generally to liquid cooling thermal management systems and more specifically it relates to a two-phase liquid cooling management system that provides economies of scale with consistent high performance through the use of a low momentum loss fluid manifold system.
2. Description of the Related Art
Liquid cooling is well known in the art of cooling electronics. As air cooling heat sinks continue to be pushed to new performance levels, so has their cost, complexity, and weight. Liquid cooling systems provide advantages over air cooling systems in terms of heat removal rates, component reliability and package size.
Liquid cooling systems are comprised of many different species. Although each specie may have unique advantages and disadvantages, they are all designed to perform the same goal: remove heat from an electronic component. Generally, liquid coolant is placed in thermal contact with a component to be cooled wherein energy is transferred from the higher temperature component to the coolant. A pump circulates the liquid through the closed system, thus allowing the cooling fluid to continuously transfer thermal energy from the component to be cooled and to a desired location. Typically, the absorbed heat is removed from the cooling fluid through the use of a heat exchanger. Species of liquid cooling systems can be lumped into two categories: single-phase and two-phase. The “phase” signifies how the cooling fluid absorbs and exchanges energy.
Single-phase liquid cooling utilizes a pure liquid for absorbing heat from the component to be cooled. Energy is absorbed by the coolant through sensible heat gains. The temperature of the coolant increases as energy is absorbed according to well known engineering formulas. An example of a single-phase liquid cooling system is described by U.S. Pat. No. 6,234,240. Due to the simple nature of single-phase fluid flow and sensible heat gains, single-phase liquid cooling solutions are fairly straight-forward to design and implement.
The preferred method of liquid cooling is two-phase. A two-phase system absorbs energy from the component to be cooled by means of latent heat gains of its fluid. The temperature of the coolant does not necessarily change, but rather part of the coolant is vaporized as energy is absorbed. The vaporized coolant is then transferred to a heat exchanger, or condenser, where energy is removed from the vapor causing it to transform back to a liquid state. An exemplary two-phase cooling solution is described by U.S. Pat. No. 5,220,804, which describes how atomization of the cooling fluid, along with vapor management within the spray module, provides high heat flux thin film evaporative cooling.
The advantages of two-phase cooling over single-phase cooling are significant. Due to the amount of energy needed to vaporize a liquid in comparison to the energy required to raise its temperature, two-phase systems provide the ability to have more compact components, require less input energy and provide higher heat removal performance than single-phase systems.
Although both single-phase and two-phase liquid cooling solutions provide many advantages over air cooling solutions, they also have drawbacks. One such drawback is that liquid cooling can be more expensive than air cooling. In the case of a single processor application, an air cooling heat sink may be comprised of an aluminum extrusion and a fan. In the case of liquid cooling, for each processor, a pump, a heat exchanger, tubing, fittings, fluid and a thermal management unit are needed. Although the performance of liquid cooling may justify the increased cost over air cooling, liquid cooling a single processor may require a cost premium.
One of the ways to reduce costs of liquid cooling solutions is to cool multiple electronic components from a single closed loop liquid cooling system. In the case of a rack full of servers, many computer systems may be chained together. The result is a significant savings through economies of scale. Chaining electronic components together can be accomplished in two ways: parallel and series connections.
With series connections, the fluid is routed from one heat generating component to another, until all units have been cooled. Although this method is largely used with single phase systems, it can also be used with two-phase systems. A significant problem with series connections is that the cooling fluid is at a different thermal state at each electronic component along the chain. As one processor may go from a max power consumption state to an idle state, that processor may create a thermal cycle for the other processors in the system. Thermal cycling reduces component reliability.
With parallel connections, the fluid is routed from the pump directly to all components to be cooled. The fluid is also removed from the thermal management units via individual parallel branches. A prior art return system is shown in FIG. 2 of the attached drawings wherein each thermal management unit has a unique fluid path. Although this type of connection is used primarily with two-phase systems, it can be used with single-phase systems as well. Parallel connections remedy the disadvantages of series connections, but it too creates challenges.
A first challenge with parallel connections using two-phase flow is that the flow of fluid can be complicated. Vapor is significantly less dense than liquid and thus the mixture can create multiple flow patterns including: annular, slug and froth. The mode of flow can be difficult to predict and tests have shown the mode of flow to have a significant impact on the performance of a cooling system.
Another problem with parallel connections is that the numerous transitions can cause system back-pressures. Back-pressures, or restrictions downstream of a spray module, can cause an increased pressure level within a spray module. Increased spray module pressures decrease vaporization and overall heat transfer rates. In addition, back-pressures create system inefficiencies as the pump must perform additional work.
Yet another problem with parallel connections is that in many computer cooling applications, the locations of particular processors may not be fixed. The parallel connections of the cooling system must be created in a fashion that provides configuration flexibility.
Thus, there is a need for a two-phase parallel liquid cooling solution capable of cooling many electronic components in non-specific configurations. It is highly desirable for such a system to provide consistent cooling performance under a wide range of conditions.